1. Field of the Invention
The present invention relates to an electrophotographic machine using a liquid developer or a dry developer and a method of printing using the electrophotographic machine.
2. Discussion of the Background
Electrophotographic machines using liquid developers have advantages not realizable by dry electrophotographic machines and so their values have been recognized again in recent years. Wet electrophotography is superior to dry electrophotography for the following reasons: (1) Small toners of submicron size can be used and so high quality can be accomplished; (2) Sufficient image concentration can be obtained with a small amount of toner which is more economical. In addition, good print quality comparable with a printing technology (such as offset printing) can be accomplished; and (3) The toner can be fixed to the paper at a relatively low temperature and so power consumption reduction can be accomplished.
However, wet electrophotography using a background liquid developer has intrinsic problems as described below. Thus, wet electrophotography has permitted dry technology to be in an unchallenged position for a long time. One of the problems with wet electrophotography is associated with the transfer step.
A first problem with the transfer process is a deterioration of the image quality. In particular, in the background technique, developer adhering to a latent image carrier is directly transferred to paper by an electric field in the transfer process. Unevenness of the paper surface varies the electric field, thus producing nonuniform transfer. Furthermore, poor transfer tends to occur due to variations in electrical characteristics of the papers used and their environment-dependence. These problems have considerably deteriorated the quality of the transferred image.
A second problem with the transfer step is an environmental problem caused by adhesion of the solvent to the paper. Where a liquid developer is transferred by an electric field, charged toner particles move through the solvent by electromigration and transfer to the paper. Accordingly, in transfer using an electric field, a given amount of solvent must be interposed between the latent image carrier and the paper. Consequently, a large amount of solvent adheres to the toned paper. This solvent partially evaporates during a fixing process which uses heat and is released out of the machine. This produces odors and adversely affects the human body if the vapor is inhaled. In addition, the paper discharged out of the machine after being fixed still contains a large amount of solvent. If an allergic user touches the paper, skin inflammation such as eczema may be caused.
Some methods for solving these problems have been proposed. These methods include first transferring toner particles containing a given amount of solvent to an intermediate transfer medium and then transferring the particles to the paper. In U.S. Pat. Nos. 5,148,222, 5,166,734, and 5,208,637, there is disclosed a method of transferring toner particles from a latent image carrier to an intermediate transfer medium by an electric field and then to the paper by pressure and heat. In Japanese Patent Publication No. 41679/1971 and Japanese Patent Laid-Open No. 280882/1987, there is disclosed a method of using pressure and/or heat for transfer to an intermediate transfer medium and also for transfer to the paper without using an electric field in the transfer steps.
It is relatively easy to fabricate the intermediate transfer medium from a material that has a smooth surface and electrical resistances that differ less among products or vary less with use. Therefore, image deterioration due to transfer is suppressed to a much greater extent than where direct transfer to the paper is performed using an electric field. Also, where transfer to the intermediate transfer medium is performed using pressure and heat, image quality deterioration is suppressed greatly.
In these proposed methods, transfer to the paper is performed by heat and pressure and so the problems encountered where an electric field transfer is used do not take place.
Furthermore, the solvent adhering to the intermediate transfer medium can be evaporated by heating or can be sucked up (e.g., suction of air) before the transfer to the paper under pressure. Consequently, adhesion of the solvent to the paper can be greatly reduced. Where transfer to the paper is performed by pressure, no solvent is necessary. Hence, such an improvement is possible.
However, these proposed methods pose the following problems in practical applications. First, to transfer toner particles to an intermediate transfer medium using an electric field, it is necessary to fabricate the intermediate transfer medium from a material having both electrical conductivity and resilience. This requirement must also be satisfied in transferring dry developer. If an excessively high pressure is applied to the developer, the adhesive force exerted between the developer and the latent image carrier is increased, making it more difficult to transfer the developer by an electric field.
In the past, therefore, the intermediate transfer medium is made of conductive rubber to achieve both resilience and conductivity. Generally, however, where conductive carbon or the like is added to give conductivity to the rubber, the resilience of the rubber is impaired and it becomes difficult to obtain a soft touch. This urges a use of a relatively hard rubber. Therefore, in order to obtain a soft touch, a latent image carrier and an intermediate transfer medium need to be disposed opposite to each other at a quite high accuracy.
This requirement needs highly accurate components, which in turn results in an increase in the cost and a deterioration in the production yield. Furthermore, the rubber swells due to petroleum-based solvent contained in the liquid developer which varies the thickness. Therefore, the operator must frequently adjust the positional relationship between the intermediate transfer medium and the latent image carrier. Hence, only few choices are available for the conductive rubber material which can minimize such swell, and thus serious difficulties arise.
The aforementioned problems associated with electric field transfer can be circumvented by transferring toner particles to an intermediate transfer medium by pressure. However, the following problems occur. In this case, the toner particles are transferred mainly because of a difference in surface energy or releasability among components. That is, the toner particles are transferred to the intermediate transfer medium only if the surface of the latent image carrier has a releasability higher than that of the intermediate transfer medium.
This releasability relationship is also needed between the intermediate transfer medium and the paper. Accordingly, a clear order of releasability must be established among the latent image carrier, the intermediate transfer body, and the paper. This greatly limits the choices for the materials of these components.
Furthermore, after long term use, the surfaces of the components are contaminated with the developer and so the states of the surface vary. This makes it difficult to maintain the order of releasability as described above. These problems lead to deterioration of the transfer characteristics, which in turn impairs the image quality.
The above-described problems are mainly for the case using liquid developers. However, most of the aforementioned problems also occur in the electrophotographic machines using dry developers. Specifically, when a developer is directly transferred from a latent image carrier to the paper by an electric field, the image quality is deteriorated. In addition, when the paper or the intermediate transfer medium is pressed against the latent image carrier, it is difficult to perform the transfer by an electric field. Where transfer is done by pressure, the order of releasability among the components becomes important, leading to the limitations on selecting materials, and thus the durability is deteriorated. These problems are common to the case where dry developers are employed.
As mentioned above, in the electrophotographic machine using the background liquid development technique, a harmful solvent contained in the solvent tends to adhere to the paper to be discharged from the machine. In transfer using an electric field, unevenness of the surface of the paper and excessively high contact pressure have deteriorated the image quality. In transfer using pressure, problems such as limitations for the choice of the materials for the components and insufficient durability take place. These problems are common to the electrophotographic machines using dry developers.